Electromagnetic clutches are well known in the prior art and may be used to control the transfer of power from an automobile engine to the refrigerant compressor of an automotive air conditioning system. The general structure of an electromagnetic clutch for an automobile air conditioning compressor is disclosed in U.S. Pat. Nos. 3,044,594 and 3,082,933.
The construction of a conventional electromagnetic clutch is shown in FIG. 1. The clutch assembly is disposed on the outer peripheral portion of annular tubular extension 2, which projects from an end surface of compressor housing 1 to surround drive shaft 3. Drive shaft 3 is rotatably supported in compressor housing 1 by bearing 4. The clutch assembly includes a rotor 5 rotatably mounted on tubular extension 2 by bearings 6. The rotor is driven by a belt coupled to the automobile engine (not shown). Rotor 5 is provided with a plurality of concentric arcuate slits 5a, 5b, forming magnetic pole face 5c. A hub 7 is fixed to the outer terminal end of drive shaft 3 extending beyond tubular extension 2. Armature plate 8 is flexibly jointed to hub 7 by a plurality of leaf springs 9. Leaf springs 9 are fixed to the outer surface of armature plate 8 by rivets 11. The axial end surface of armature plate 8 faces pole face 5c of rotor 5 with a predetermined axial air gap G therebetween. The axial end surface of armature plate 8 is provided with concentric arcuate slits 8a, forming pole face 8b. Slits 8a are positioned to be opposite the midway point between slits 5a, 5b on pole face 5c.
Electromagnet 10 is mounted on compressor hoursing 1 concentric with drive shaft 3. Electromagnet 10 includes an electromagnetic coil 101 disposed within annular hollow portion 5d of rotor 5 and is surrounded by an air gap. When coil 101 of electromagnet 10 is energized, pole face 8b is attracted to pole face 5c. Thus, drive shaft 3 rotates as rotor 5 is turned by the engine. If coil 101 of electromagnet 10 is not energized, pole face 8b of armature plate 8 is separated from pole face 5c of rotor 5 by the recoil strength of leaf springs 9. Rotor 5 still rotates in response to the engine output, but drive shaft 3 is not turned.
In the above construction of the electromagnetic clutch, magnetic flux M, which is produced around electromagnet 10 by the energizing of electromagnetic coil 101, passes through a magnetic passageway formed within electromagnet 10, rotor 5 and armature plate 8. Since magnetic flux tends to follow the shortest path through the magnetic passageway, the flux M from pole face 5c of rotor 5 passes through rivet 11, rotor 5 and armature plate 8 in a zig-zag manner, as indicated by the dotted line in FIG. 1. If flux M follows this path through the magnetic passageway, a frictional torque small in comparison with the strength of the magnetic field is generated.